Plasma processes are used for numerous fabrication steps in various device manufacturing applications such as semiconductor integrated circuit, data storage device (heads and media), and flat-panel display manufacturing. Typically, plasma processes (also known as plasma-enhanced or plasma-assisted processes) are used for physical-vapor deposition (PVD), plasma-enhanced chemical-vapor deposition (PECVD), dry etching, wafer cleaning (or surface preparation), in-situ chamber cleaning, and plasma-immersion ion implantation (also known as plasma doping) applications. Conventional or prior art methods of plasma generation employ one or a combination of several techniques. Various plasma generation techniques include parallel-plate capacitive discharge, microwave discharge (including electron cyclotron resonance or ECR plasma), hollow cathode discharge, and inductively-coupled plasma (ICP) sources.
The high-density inductively-coupled plasma or ICP sources have recently received a significant amount of attention due to their superior process performance, throughput rate, and control capabilities. ICP sources can provide high-density (n.sub.p values ranging from 1.times.10.sup.11 cm.sup.-3 to over 5.times.10.sup.12 cm.sup.-3) plasmas using fairly simple inductive radio frequency (RF) excitation. Advanced ICP source designs are capable of producing fairly high plasma densities (corresponding to the plasma electron density or n.sub.p) even larger than 1.times.10.sup.13 cm.sup.-3. The RF source frequency is typically in the range of 1 to 30 MHZ (with a preference for 13.56 MHZ). RF frequencies in the low end of this range result in reduced induced RF voltages across the ICP antenna. This reduces the risk of capacitive coupling as well as sputtering of the inner process chamber and ICP source walls near the ICP antenna. Lower ICP source frequencies, however, result in reduced plasma densities and larger RF matching network components. On the other hand, higher RF frequencies can provide superior plasma densities and can be effectively coupled to the plasma load using more compact RF matching network components. However, precautions must be taken to ensure that no chamber wall sputtering occurs due to the relatively high induced RF voltages that arise across the antenna. Higher induced RF voltages across the ICP source antenna can increase the risk of capacitive coupling and raising the plasma potential.
One advantage of ICP over conventional parallel plate plasma is its ability to control the plasma density and ion energy (for the ion flux arriving at the substrate in process) independent of each other. The plasma density is primarily controlled by the applied RF current or power delivered to the ICP source antenna, whereas the mean ion energy control is performed by an applied RF bias to the substrate or wafer. The substrate may be a semiconductor wafer (e.g., silicon), a data storage substrate (AlSiMag or AlTiC), a photovoltaic substrate (e.g., polysilicon or silicon), or a flat-panel display substrate (e.g., glass).
Various types of ICP source designs have been proposed in prior art. These include spiral coil antenna designs, helicon wall source designs, and cylindrical coil antenna source designs. However, all the prior art ICP designs share a common constraint or limitation which makes them unable to control or adjust the plasma uniformity profile in real time. The prior art ICP sources are primarily based on single-zone designs and employ single-coil antenna structures with a single RF plasma excitation source. The basic prior art designs mostly employ either a cylindrical or cone-shaped coil around a quartz chamber (such as a quartz bell jar) to generate a large-volume plasma or a planar spiral coil above a dielectric plate (outside the vacuum chamber) to generate a so-called planar plasma. The spiral coil ICP design often uses a flat spiral coil, but provides the options to contour the surface topography of the ICP antenna dielectric housing and/or the antenna coil itself for improved plasma uniformity.
The spiral coil design possesses certain technical advantages, but also has serious limitations. The spiral coil design allows placement of the antenna above a vacuum dielectric plate on the atmospheric side or within the vacuum chamber using an epoxy encapsulation. One can provide a capability to reduce the induced RF voltage across the spiral coil by placing a few capacitors in series with the spiral coil loops. This is not a trivial implementation task since the antenna coil is usually made of water-cooled aluminum or copper tubing. Insertion of the series capacitors may require breaking the tubing water flow by insertion of an in-line metal-to-ceramic insert. Unfortunately, this results in added structural complexity and increased equipment cost. The ICP sources with cylindrical coils around the electrically insulating plasma source or process chamber require an electrically insulating process chamber or plasma source wall material such as quartz tube or aluminum oxide tube used in some source designs such as the helicon plasma sources. These bulk ICP sources can suffer from plasma non-uniformity problems and usually require a multipolar magnetic bucket inserted between the plasma source chamber and the process environment to generate an expanded uniform plasma. This, however, results in reduced processing throughput due to reduced plasma density and ion flux density at the substrate. Moreover, these sources may generate contaminants and particulates due to sputtering of the plasma source chamber wall material near the excitation RF antenna.
The ICP coil is usually driven by a 13.56 MHZ RF source. The RF current also induces an RF voltage across the antenna coil. In order to eliminate any electric field induced arcing or chamber sputtering, the amount of induced RF voltage must be minimized. This condition places a limit on the maximum allowable excitation coil inductance or the number of coil turns. Moreover, for a given coil design (e.g., a given number of turns or inductance) there is an upper limit on the maximum allowable RF source frequency. In addition, for a given conventional ICP source design and a specified excitation RF frequency (e.g., 13.56 MHZ), there is a limit on the maximum allowable RF power delivered to the ICP antenna in order to ensure minimal chamber or plasma source wall sputtering and reduced process contamination. The prior art designs for ICP coils, therefore, mostly suffer from plasma process nonuniformity problems, are not easily scalable for larger wafer processing, and have a relatively narrow useful process window (in terms of RF power, pressure, etc.). The conventional ICP designs do not provide any direct method of real-time plasma uniformity control without compromising the significant process state or substrate state parameters.
Advanced plasma fabrication processes require excellent plasma density and ion flux uniformity control over the entire wafer surface. Plasma uniformity requirements in high-density plasma sources are dictated by both process uniformity requirements and device damage considerations. Typically, the plasma nonuniformity must be less than 5% (3-sigma value) to ensure damage-free uniform processing. Many conventional ICP source designs fail to meet these stringent process uniformity requirements for various plasma processing applications.